The present invention relates to light guides for Liquid Crystal Displays (LCDs), and in particular to the use of a Fresnel lens to redirect light from a light source.
LCD displays are used in televisions, computer monitors, mobile phones and other devices. LCD displays use a back light module to illuminate the LCD display. There are two popular methods for the placement of the light source. For large screens the light source is placed behind the LCD display. For smaller screens the light source is placed on the side of a light guide. The most common light source used in television is CCFL (cold cathode fluorescent lamp), which requires a high voltage source. A more energy efficient light source is the Light Emitting Diode (LED). However, there are obstacles to using LEDs.
One of these obstacles is illustrate in FIG. 1. An array of LEDs 102 is mounted on a printed circuit board (PCB) 101 and is placed behind the LCD screen 103. Due to the divergence angle of the LED light source, the LCD screen 103 must be placed at a distance from the LED in order to have uniform illumination of the LCD screen. This defeats the goal of having a thin display.
One approach to making a thinner display is to use side illumination, as illustrated in FIG. 2. A typical back light module for LCDs used in desktop and laptop computer displays have side illumination as shown in FIG. 1. Side illumination is used rather than having the LEDs behind the screen in order to reduce the thickness of the display. However, the current method of coupling LED light into a light guide is not efficient enough for their use in a large screen television. Thus, fluorescent light (CCFL) is typically used for large screen TVs, while smaller displays, such as mobile phones, can use LEDs.
As shown in FIG. 2, a light source 110 injects light into a light guide (waveguide) structure 112, which is typically a clear slab of plastic or acrylic. Light source 110 can be a cold cathode fluorescent lamp (CCFL) or a light bar with a number of LEDs. Light source 110 can be enclosed by a cylindrical reflector 114 to focus the light into the light guide 112. Due to the higher index of the light guide material, a light ray such as ray 116, which is incident on the top surface of the light guide with an angle larger than the critical angle, is reflected and propagates toward the bottom of the light guide. Light ray 118, which is incident at an angle smaller than the critical angle, will leak out of the light guide. In order to facilitate the coupling of light out of the waveguide, some structures are put on the bottom surface 120 of the light guide.
FIG. 3 shows an example of the structures 201 used on surface 120 of FIG. 1. The circles 202, 203 indicate the locations of highly reflective particles on the bottom surface of the light guide. These particles can be made either of reflective paint or structures such as hemispheres. In order to achieve uniform distribution of the light emitting from the light guide, fewer particles are placed on the side closer to the light source. Otherwise, too much light would be reflected out from the side close to the light source, leaving too little light to propagate to the right and be reflected out the far side of the light guide. The size and density of the circles in FIG. 2 symbolically illustrates a variation of the reflective particles on the bottom surface. Unfortunately, with the light source placed at the side of the light guide, most of the light coupled out from the waveguide propagates almost parallel to the top surface of the light guide. This is highly undesirable, since the viewer is looking in a direction normal to the surface of the light guide.
FIG. 4 illustrates one of the techniques to overcome this difficulty in a structure with a light source 301, cylindrical reflector 302 and reflector 303. A diffuser 304 is put on top of the light guide. Light rays incident on the diffuser are scattered over a larger cone angle 305. Hence there will be more light propagating closer to the normal direction of the light guide. This diffuser also makes the light emerging from the waveguide more uniform. However, even with the diffuser, the light emerging from the light guide is still not quite normal to the display surface.
FIG. 5 shows a structure with two light sources 501, one at each end of the light guide structure, each with its own cylindrical reflector 502. A reflector 503 and diffuser 504 are shown. A prism film 505 is added to the light guide structure. The prism film only allows light rays within certain angles to be passed through. The light rays emerging from the prism film will be within +/−30 degree with respect to the normal of the light guide surface, as shown by light rays within angle 506. As can be seen, such an illumination system is highly light inefficient and expensive in cost due to the multiple layers of film needed to correct for the illumination distribution. In order to provide sufficient illumination, often light sources on the two sides are used as shown, although alternately a light source on a single side can be used, such as for a smaller display.
There are many examples of improvements on this basic back light module. For example, U.S. Pat. No. 5,392,199 and U.S. Pat. No. 5,647,655 show a reflecting plate below a light guide, and a diffusion layer above it. The ends of the cylindrical reflector around the light source are bonded to the light guide. U.S. Pat. No. 5,779,337 shows a light guide with a plurality of projections or grooves arrayed on the light emitting (top) surface to cause more light to be emitted. U.S. Pat. No. 6,115,058 shows an LCD using polarizing filters and a Fresnel lens 201 bonded to the back of the LCD. U.S. Pat. No. 6,710,829 shows a press to generate a plurality of recesses of different sizes on the illuminating plate of a back light plate. U.S. Pat. No. 7,160,018 shows a back light plate with an array of prisms on the front or top, and reflective dots on the bottom surface. U.S. Pat. No. 7,270,466 shows a bottom reflecting surface with different sized and spaced protrusions on the two sides of the reflecting surface.
A number of other patents and published applications discuss the use of a Fresnel lens structure in a backlighting application. Such a lens can be used to disperse light from an LED into a light guide for the backlight. US Published Application No. 2006-0256578 describes a Fresnel structure for reflecting light from an LED into a light guide. This is also shown in U.S. Pat. No. 6,935,764. US2007-041215 shows a Fresnel lens for corners. U.S. Pat. No. 5,579,134 shows a Fresnel lens for a top surface. US2002093742 shows a microfresnel lens array on top of a light guide, with an array of reflective protrusions below, with the protrusions reflecting light from the side to the microfresnel lens array on axis (straight on). U.S. Pat. No. 6,935,764 shows a Fresnel lens for dispersing the LED light where it enters the light guide. U.S. Pat. No. 6,883,934 shows a Fresnel lens over the LED. U.S. Pat. No. 5,851,062 and U.S. Pat. No. 7,018,088 show a prism above (in front of) the LCD. U.S. Pat. No. 7,226,196 shows deformities on the top and bottom layers of a light guide.
U.S. Pat. No. 6,196,691 describes a single, ruled diffraction structure on the bottom of the light guide to couple light out. The diffractive structure has a constant period.
It is an objective of this invention to provide an efficient way to couple light into a light guide for use in large screen television. This same method can also be used in small screen displays such as the ones used in mobile phones or other personal devices. It is also an objective to remove the need for the multiple layers, with their added expense and size, such as the prism and diffuser layers.